368 ALASKAN GLACIER STUDIES
decreased to 30 feet to the mile. The average bottom slope for 18 miles is, therefore, concave upward and, like a stream course, steepest at the head. The lines of soundings across the fiord opposite the mouth of Blackstone Bay, and east of Port Wells, show that it has a broad flat bottom; and the typical steep walls of the visible portion of the fiord are continued below sea level, a depth of 1170 feet being found a quarter mile from the northern shore just west of Pt. Pigot.
There are two conspicuous interruptions of the slope of the fiord bottom (Fig. 54), one near the delta of Billings Glacier, where the water is only 540 feet deep, thus rising 216 to 264 feet above the fiord bottom on either side. The other is opposite the mouth of the cove of Seth Glacier, where the water is only 666 feet deep, thus rising 392 to 390 feet above the adjacent bottom. These may be either submerged moraines or ledges between rock basins in the fiord bottom, due to glacial erosion, and information is not available for determining between these two possibilities. It is noteworthy that, as in other fiords of Prince William Sound, each of these irregularities comes near where the trunk glacier formerly received a tributary.
In addition to the visible hanging valleys along the fiord walls above sea level, there are probably also submerged hanging valleys. The cove of Seth Glacier on the northern side of the fiord, the cove southwest of it, on the opposite side of the fiord, and a number of smaller indentations are suspected to have this relationship, though soundings were not made. Blackstone Bay may have a slightly discordant relationship to Passage Canal (96 feet), though this is thought doubtful, because the combined Tebenkof and Black-stone Bay Glaciers must have nearly or quite equalled the former Passage Canal Glacier both in volume and in erosive power. '
Sparseness of Glacial Deposits. There is conspicuous absence of glacial deposits in Passage Canal. The outwash gravels of the Billings Glacier valley train, with the large delta at its terminus, and the outwash flat at the head of the fiord, near Portage Glacier, are the only deposits of any size above sea level, although there is the usual irregularly distributed veneer of ground moraine. Mention has already been made of the 540 and 666-foot submarine ridges, which may represent submerged moraines due to halts of the southwestward retreating ice tongue of Passage Canal if they are not rock swells associated with eroded basins.
Vegetation. Mature forest at sea level extends up to the very head of this fiord, and there is no suggestion of recent glacial expansion excepting the small barren zone of the cascading glacier and the detached, moraine-covered ice block in front of Learnard Glacier. These are but minor exceptions, and the forest trees, which attain a diameter of a foot or two, show that the general glaciation was centuries ago.
This fiord is just east of Blackstone Bay, and might be considered a southward extension of Port Wells, across Passage Canal. It is lú to 4 miles wide, 12 miles long, and its head is within 4 or 5 miles of the Applegate Arm of Port Nellie Juan fiord to the south. Glacial erosion has doubtless given Cochrane Bay much of its'present form and depth, the water being 1428 feet deep near its mouth which joins Passage Canal with accordant grade, as does Port Wells to the north.
Whidbey in 1794 sketched the outlines of this bay; and Vancouver speaks of it as